The present invention relates to an X-ray tube apparatus and, more particularly, to an X-ray tube apparatus having a rotating anode X-ray tube.
Generally, an X-ray tube apparatus is employed for medical treatment in the form of, for example, an X-ray diagnosis. The X-ray tube apparatus for use in medical treatments, including the examination of the stomach, uses a rotating anode X-ray tube. This rotating anode X-ray tube has a vacuum envelope, in which a cathode assembly and an anode target are received. The anode target has a target disk. The target surface of this target disk and the cathode assembly are disposed in a manner that they are offset from the tube axis of the vacuum envelope and that they oppose each other. The target disk is connected to a rotor, which is driven to rotate by electromagnetic induction produced from a stator provided outside the vacuum envelope.
The anode assembly of the above-mentioned rotating anode X-ray tube has a focussing electrode, which is formed with a focussing dimple. Within this focussing dimple, a tungsten coil filament is provided which is intended to emit electrons. Generally, the electric potential which is applied to the filament is the same as that which is applied to the focussing electrode. Therefore, the electrons emitted from the filament are focussed on the target surface by the electrostatic field in the focussing dimple.
In this cathode assembly, however, a part of the coil filament is allowed to project into the focussing dimple of the focussing electrode. This is because the coil filament must be used within a temperature limited current range and, at the same time, the electric field should be intensified in the neighborhood of the filament. By protruding a part of the filament, the equipotential surface in the vicinity of the filament has a configuration which protrudes toward the target surface at the central portion of the filament. On the other hand, the electrons emitted substantially from side walls of the filament are directed sidewardly of the focussing dimple due to the electric field in the zone between a bottom portion of the focussing dimple and the filament. At the same time, they are directed toward the center of the focussing dimple due to the concaved electric field in the vicinity of the opening end of this dimple, and thus are focussed. Accordingly, the electrons emitted from the side walls of the filament and the electrons emitted from the central portion of the filament can not be focussed in the same spot. In other words, the loci of both electrons emitted from the two opposed side walls of the filament intersect each other on the center axis of the electron beam. When almost all of the electrons have been focussed on the target surface, the electron density distribution as viewed about a portion of the target surface including the center axis of the electron beam is twin-peaked.
In the cathode assembly having the above-mentioned construction, the electrons emitted from the filament can not be focussed, by the focussing electrode, onto a sufficiently small focal area. For this reason, the use of a small filament is required for obtaining a small focal area on the target surface. With such a small filament, however, the electrons are not emitted therefrom with a sufficiently high density unless the temperature of the filament is high. Therefore, the conventional rotating anode X-ray tube has a problem in respect of the limitation of tube current.
Further, it is difficult to direct the electrons towards the anode target, so that it is impossible to obtain a minute focal area. Further, the electron distribution has no sharpness, so that it is impossible to obtain a desired distribution of electrons. For this reason, it is difficult to obtain both a sufficiently high resolution, and a decrease in the maximum value of rise of the temperature on the anode target, due to the incidence of electrons, to thereby cause an increase in the amount of the electrons incident thereupon. Where the projection image is prepared by using the X-rays generated from the anode target, these drawbacks become obstacles to the decrease in photon noises as well as the increase in resolution, failing to obtain a sufficiently clear image.
The use of a flat-plate like cathode filament is contemplated as a method of removing the above-mentioned drawbacks. An example wherein such a filament is used is disclosed in Japanese Patent Disclosure No. 68056/80.
In the X-ray tube proposed in said literature, a cathode filament consisting of a flat strip-like plate is used. The central portion of this cathode filament is flattened by bending both end portions thereof. The cathode filament is formed with leg portions at both its end portions. The leg portions of the cathode filament are mounted on filament supporting struts, respectively. When it is directly heated by passing electric current, the cathode filament emits electrons mainly from its central portion. In this proposal, a focussing electrode whose focussing dimple is small in depth is used. The electrons emitted from the cathode filament are focussed by means of the focussing electrode. The equipotential curve in the vicinity of the focussing electrode has a gentle curve at the central part of the focussing dimple. The anode target is kept high in positive potential relative to the cathode filament and focussing electrode. It is located at a position which is spaced from the focussing electrode by a distance equal to a focal distance of an electron lens thereof.
The above-mentioned conventional example, however, has the following drawbacks. First of all, limitation is imposed upon the focussing of electrons. That is, it is known that the width of spread of the electrons on the anode target, W, is given in the following formula, ##EQU1## Where Vo represents the initial velocity energy of electrons, and Va represents the anode potential. Actually, however, when, for example, f=15 mm, Vo=0.2eV, and Va=30 keV are substituted into the above formula, W=0.08 mm. Namely, a sufficiently small focal area is not obtained.
The second drawback is that the loci of the electrons emitted from the side walls of the cathode filament are greatly different from those of the electrons emitted from the central portion thereof. That is to say, a sub-focal area is formed in the distribution of electrons on the anode target. This is because the loci of the electrons emitted from the end portions of the filament are affected by the equipotential curve in the area very near to the surface of the filament. The equipotential curve in such an area, i.e., the gap zone between the end of the filament and the focussing electrode is concaved. Accordingly, in that area, a local concave lens is formed. For this reason, the loci of the electrons emitted from the end portions of the filament come near to the walls of the focussing electrode as compared with a case where the equipotential curve is uniform. The focal length relating to the electrons emitted from the end portions of the filament is smaller than the focal length relating to the electrons emitted from the central portion of the filament. This is because the curvature of the equipotential curve within the focussing electrode becomes greater in those portions of this electrode near to its walls than in the central portion thereof. In the X-ray tube of this proposal, therefore, a sub focal area is formed on the target surface, failing to obtain a sufficiently high degree of focussing. Where the value of electric current is great, the spread of electrons on the anode target has a width due to the space charge which is greater than the width expressed in the above-mentioned formula.
In the case of making the electric potential of the focussing electrode equal to that of the filament and, under this condition, increasing the depth of the focussing electrode to make the focal length small to thereby increase the focussing effect, the electric field becomes weak in the zone near to the filament. Further, in such a case, the space charge limiting diode is formed in said zone. Thus, the value of electric current is varied corresponding to the anode potential. Further, where the anode voltage is around 30 kV, it is sometimes possible that a current value of 10 mA or more is not obtained.
The proposal also discloses the technique of putting a focussing electrode (or another electrode having a shallow focussing dimple at a position slightly forwardly spaced from the focussing electrode), and applying a bias voltage to it, which voltage is higher than a voltage of the filament. This technique, however, has a drawback in that the focusability of the electron beam is decreased in the longitudinal direction of the filament.
Further, in the conventional flat filament, when the temperature of the filament is increased by passing electric current therethrough, the filament is thermally expanded, so that the central portion of the flat filament, i.e., the electron emission surface is greatly curved in such a manner as to protrude toward the target surface. As a consequence, the electron emission surface is greatly displaced relative to the target surface. Thus, the conventional filament is low in reliability and is defective in that the passing of electric current through the filament does not enable a stable tube-current characteristic to be obtained.